Long hair collapses with soft body physics in animation
Probably the first thing people find when they enable soft body is that it all collapses in on itself. You need to stiffen it up a bit to have it maintain it's structure.

The first thing I would do is enable Stiff Quads and turn up Bending to 3, damp to 20 (under soft body edges) and damping to 12 (under soft body goal).

You will need to experiment more to get it right but that should get you closer.

You might find this video helpful in explaining some of the options.